Telephone system



Sept. 10, 1935. .1. BAUMFALK TELEPHONE SYSTEM Filed April 5, 1934 2 Sheets-Sheet 1 INVENTOR J. BAUMFALK ATTORNEY Patented Sept. 10, 1935 UNITED STATES PATENT OFFICE TELEPHONE SYSTEM Application April 5, 1934, Serial No. 719,073

18 Claims.

The invention relates to telephone systems and more particularly to systems employing automatic switches for establishing talking connections between calling and called subscribers stations.

The object of the invention is to provide an improved two-way trunk circuit for use in automatic telephone systems, to eliminate the difficulties encountered in the use of two-way trunks between oflices due to differences in ground potential, and to increase the eificiency of two-way trunks by eliminating the possibility of trouble or delay due to the simultaneous seizure of such a trunk at both ends.

One feature of the invention is control apparatus associated with a two-way trunk, which apparatus comprises a relay, the winding of which is normally bridged across the conductors of the trunk for response to seizure of the trunk at the distant end, means efiective upon seizure of the trunk for use on an outgoing call for disconnecting the winding of the relay from the trunk and for repeating selective impulses over the trunk, and means efifective upon operation of the bridging relay "for disconnecting the winding of the relay from the trunk and operatively connecting the trunk to an incoming selector which is individual to the trunk.

Another feature of the invention is a two-way trunk between automatic telephone ofiices arranged to transmit a busy signal to the calling station in the ofiice in which the trunk is last seized, in case the trunk is seized at both ends, and to complete the call incoming from the oifice in which the trunk is first seized.

A further feature of the invention is an arrangement for giving each of the calling subscribers a busy signal in case the trunk is simultaneously seized at both ends, and for completing the next succeeding call incoming from the office at which the trunk is first released.

and other features of the invention will be more readily understood by considering a telephone system in which the invention is embodied. One such system is illustrated in the drawings which form a part of this specification. The invention is, of course, not limited in its application to the system shown in the drawings but may be applied generally to systems employing automatic switches in the completion of interofi'ice connections.

Referring to the drawings:

Fig. 1 represents schematically a central office Ol in which AI is a subscriber's station connected by line Iill to terminals in the back Qi is line finder switch LFI; SI is a first selector switch permanently connected to the line finder LFI, TI is trunk apparatus associated with the conductors I is and I29 of a two-way interoffice trunk T, ISI is an incoming selector switch individual 5 to the trunk T, Cl is a connector switch, and BI is a subscribers station connected by line I5I to terminals in the bank of switch CI; and

Fig. 2 represents schematically a central office O-2 to which the two-way interoffice trunk T 10 extends; A2 is a subscribers station connected by line ZBI to terminals in the bank of line finder switch LFZ, S2 is a first selector switch permanently connected to the line finder LF2, T2 is trunk apparatus associated with the conductors l5v H9 and 220 of trunk T, 182 is an incoming selector switch individual to trunk T, C2 is a connector switch, and B2 is a subscribers station connected by line 25I to terminals in the bank of switch C2.

The system represented by the drawings is one in which the subscribers stations include dial impulse senders for use in controlling the selective operation of automatic switches to establish desired connections. The line finder, selector and connector switches are all of the well known two motion step-by-step type. For a detailed description of the structure of such switches and the operation of such switches when used as selectors or connectors, reference may be had to pages 53 to 65, inclusive, of the 2nd edition of Automatic Telephony by Smith and Campbell published in 1921. For a description of the operation of such switches when used as line finders, reference may be had to the patent to H. Hovland No. 1,711,682 granted May "I, 1929. Each of the line finders, first selector and connector switches is represented by a set of brushes and a single set of terminals, all associated apparatus not necessary to a full and clear description of this invention being omitted. While not shown in 0 the drawings, incoming trunk impulse repeaters might, if necessary, be inserted between the trunk equipments TI and T2 and the incoming selectors ISl and 1S2 respectively.

Assume a call to have been originated at station AI, line IUI to have been found by line finder LFI, selector SI to have been operated in response to the impulses, created by the dialing of the first digit of the number of the station being called, to select the level which gives access to a group of trunks including the trunk T, and the selector SI to have hunted, found and seized an idle set of terminals, all in well known manner. Assuming the seized terminals 20 be represented by the set shown in the 55 drawings as connected to conductors I01, I08, I09 and individual to the trunk T, the line relay ill? of trunk equipment TI is operated by the current in a circuit traced from battery through the right winding of this relay, right back contact of relay I I I, conductor I08, terminal and brush Hi5 of selector SI, brush I03 and terminal of line finder LFI, over line IOI and through the instrument at station AI,.terminal and brush I02 of line finder LFI, brush I04 and terminal of selector SI, conductor I01, outer left back contact of relay I I I, through the left winding of relay I I2 to ground. At the same time that relay IE2 is being operated, relay I29 is being operated by current in a circuit from battery through the winding of relay I29, back contact of relay I25, right back contact of relay I30, conductor I2I, left back contact of relay I23, to the incoming sleeve conductor I09 and thence through the terminal and brush I06 of selector SI to ground at the release relay (not shown) of the selector switch. Relay II2 closes a circuit for operating relay I I6. Relay I I6 closes a circuit for operating relay I24 and connects ground to conductor I2I, thence through the back contact of relay I23 over conductor sleeve I09 to the terminal and brush of selector SI to serve as a guarding potential for the trunk T and a holding potential for the selector SI and line finder LFI after the temporary ground connection. is opened by the deenergization of the release relay of the selector, in well known manner. Relay I24 also connects ground to conductor I2I and closes a circuit for operating relay I26. The operation of relay I26 causes the release of relay I29.

Normally, the right winding of relay I2'I of equipment TI is bridged across conductors II 9 and I29 of trunk T and the left winding of relay 22? of equipment T2 is bridged across conductors I 29 and I20 of trunk T. When relay I29 operates, as above described, the right winding of relay I21 is disconnected from the conductors of trunk T and a circuit is closed from battery through the left winding of relay I25, right front contact of relay I29, left back contact of relay 1, right back contact of relay II8, over conductor II9 of trunk T, conductor 2 I9, left back contact of relay 2I8, right back contact of relay 2II, left back contact of relay 229, left winding of relay 227,

right back contact of relay 229, left back contact of relay 2H, right back contact of relay 2I8 over conductor 220 of trunk T, conductor I20, left back contact of relay II8, right back contact of relay III, left front contact of relay I 29, through the right winding of relay I25 to ground; relay I25 at one end of trunk T and relay 221 at the other end of trunk T are both operated by the current in this circuit.

In operating, relay I25 closes a circuit from battery through its left winding, winding of relay II'I, front contact of relay I25, conductor I22, to ground at the middle front contact of relay I24; relay I25 is locked and relay I I1 is operated by the current in this circuit. Relay II'I disconnects the windings of relay I25 from conductors II9 and l29 and closes a bridge across these conductors which may be traced from conductor H9 through the right back contact of relay I I8, right front contact of relay Ill, inner left front contact of relay IIIl, impedance II4, Winding of relay II3, left front contact of relay II2, left back contact of relay US to conductor I20. At the other end of trunk T, relay 227 closes a locking circuit from battery through the windings of relays 2I8 and 230 in parallel, through the front contact and right winding of relay 22?, and through resistance 228 to ground. In operating, relay 230 disconnects the winding of relay 229 from conductor 22I and connects ground over conductor 22I through the back contact of relay 223, over conductor 2 .29, to the sleeve terminal of trunk T in the banks of all selectors such as S2 having access to this trunk, thereby guarding against seizure by any selector in office O2. In operating, relay 2I8 disconnects conductors 2I9 and 220 of trunk T from the left winding of relay 22! and extends trunk T over conductors 234 and 235 to the incoming selector IS2 and through the windings of the line relay 23? of this selector to ground and battery respectively. There is therefore a closed metallic circuit over the conductors of trunk T which circuit includes the windings of the line relay 237 cf the incoming selector at one end of the trunk and the winding of the polarized relay I I3 at the other end of the trunk; the current in this circuit is effective to operate the line relay 23? but is not effective to operate relay H3. The operation of the line relay 231 of the selector IS2 is followed by the operation of the release relay 23B, thereby connecting ground over sleeve conductor 236 through the right front contact of relay 239 to hold relays 239 and H8 operated and to short--circuit the right winding and thereby cause the release of relay 22?.

When the calling subscriber at station AI dials the next digit of the number of the station being called, the line relay I52 of trunk equipment TI is alternately released and reoperated in response to the interruptions of the line circuit by the dial I00. The first release of relay I52 closes a circuit for operating the slow-to-release relay II5. Each time relay H2 releases and reoperates the bridge across conductors IIS and I20 is opened and closed at the left contacts of relay I I2, thereby causing a corresponding release and reoperation of the line relay 237 of selector IS2 in office 0-2. Relay H5 remains operated while impulses are being received as does also the release relay II 6. With relay I55 operated, the winding of relay H3 and impedance H4 are short-circuited by the left front contact of relay I I5 to improve the transmission of impulses over trunk T. The response of relay 231 to the impulses received over trunk T causes the operation of selector IS2 to select the level of the terminal bank corresponding to the digit dialed. The selector IS2 then automatically selects and seizes an idle set of terminals leading to a connector switch such as C2, the conductors 234 and 235 being extended through brushes 2: and 242 to the windings of the line relay 244 of this connector. Line relay 244 operates, closing the circuit for operating release relay 245. Relay 245 connects a holding ground to the sleeve conductor 24? thence through the terminal and brush 243 of switch IS2 to conductor 235. Upon extension of the connection through the brushes of selector IS2 the line relay 237 of this selector is disconnected, relays 237 and 238 being thereby released.

When the last two digits of the number of the station being called are dialed by the subscriber at station AI, relay II2 of trunk equipment TI responds, the impulses being repeated over trunk T to cause the alternate release and reoperation of line relay 244 of connector switch C2. The brushes of the connector are thereby selectively positioned to engage the terminals of the called line which is represented by line 25I leading to station B2. The line 25I having been tested and found idle, ringing current is transmitted over the line from the connector switch to signal the subscriber at station B2 in well known manner. When the subscriber at station B2 answers the call, relay 245 operates to interchange the connections between the conductors of trunk T and the windings of relay 244, the polarized supervisory relay H3 of trunk equipment TI being operated by the resulting reversal of the current through its winding. Relay H3 closes a circuit for operating relay III and relay Hi interchanges the connections between the windings of relay H2 and conductors I01 and I08, the resulting reversal of current over these conductors being used to control meter operation or any other desired supervisory function. The inner left front contacts of relay I ll prevent the opening of the bridge across the conductors of trunk T due to a temporary release of relay I I2. The talking circuit includes condensers H0 of trunk equipment TI and condensers 248 of connector switch C2.

When the connection is released by the calling subscriber, relay H2 of trunk equipment TI releases causing the temporary reoperation of relay H5 and the release of relay H6. If relay H3 is not operated, the release of relay H2 also causes the immediate release of relay 244 of the connector switch C2; but if relay H3 is operated at the time relay H2 releases, the opening of the bridge across the conductors of trunk T awaits the release of relay H5 at which time relays I I3 and III in office O-I and. relay 244 of connector C2 are released. In any case the. release of relay H6 is followed by the release of relays H5, I24 and I28. The release of relay I24 causes the release of relays I25 and H1, the trunk equipment TI being thereby restored to normal ready for use on a call in either direction over trunk T.

The release of relay 244 of connector C2 causes the release of relay 245. The release of relay 245 disconnects the holding ground from conductors 241, thereby causing the return of selector 1S2 to normal and causing the release. of relays 2I8 and 239 of trunk equipment T2. The release of relay 23c disconnects the busy ground from conductors 22! and 249 so that trunk equipment T2 is now ready for another call in either direction. With relay 245 released the connector C2 is restored to normal in usual manner as soon as the subscriber at station B2 replacesthe receiver.

Assuming now that the subscriber at station A2 originates a call, that the line 2M is thereupon extended by the operation of line finder LFZ to the selector S2, and that in response to the dialing of the first digit of the number of the station being called the selector S2 selects the terminals which are individual to trunk T. If trunk T is not at the same time being seized in office O-I, relays M2, 255 and 224 are operated in succession; and relays 229 and 225 of trunk equipment T2 and relays I21, H8 and I24 of trunk equipment TI are operated in succession in the manner that the correspondingly numbered relays of trunk equipments TI and T2 were operated as described above on the call from station AI to station BI. Upon dialing the remainder of the number of the station being called, the impulses will be repeated by relay 2 I 2 over trunk T to control the selective operation of selector ISI and a connector CI to complete the connection with the called line which is represented by line I5I of the station BI.

It may be, however, that trunk T is being seized by a selector in office O-I at the same time that it is being seized by a selector in oifice 0-2. If, in such a case, relay I25 at one end of the trunk is operated at the same time that relay 1 229 is operated at the other end of the trunk, relay I25 is connected to conductors H9 and I25 and relay 225 is connected to conductors 2I9 and 222. Neither relay I25 nor relay 225 operates, and when in office O-l relay I25 operates as above described, a circuit for operating relay I23 is closed from battery through the winding of relay I23, front contact of relay I26, back contact of relay I25, to ground at the middle front contact of relay I24; and when in office 0-2 relay 226 operates in similar manner, a circuit for operating relay 223 is closed from battery through the winding of relay 223, front contact of relay 225, back contact of relay 225 to ground at the middle front contact of relay 224. When relay I29 releases, the right winding of relay I21 is again bridged across conductors H9 and I25 in ofiice O-I; and when relay 229 releases the left Winding of relay 221 is again bridged across conductors 2I9 and 220 in office 0-2. The operation of relay H3 in office 0-! connects ground from the middle front contact of relay I24, over conductor I22, through both right front contacts of relay I23 to sleeve conductor I 89 to hold selector SI and line finder LFI and connects one winding of the tone transformer I33 through condenser I32 and the left front contact of relay I23 to one of the talking condensers I It thereby transmitting a busy tone to the calling subscriber in office O-I. The operation of relay 223 in office 0-2 in like manner connects ground from the middle front contact of relay 224, over conductor 222, through both left front contacts of relay 223 to sleeve conductor 209 to hold selector S2 and line finder LFZ and connects one winding of tone transformer 233 through condenser 232 and the right front contact of relay 223 to one of talking condensers 2Ill thereby transmitting a busy tone to the calling subscriber in office 0-2. There is no talking circuit between the two calling subscribers since neither of relays H1 or 2H have been operated. When each one of the calling subscribers releases the connection, the corresponding trunk equipment, TI or T2, is restored to normal rendering that end of the trunk again selectable for a new call. Thus the release of the connection by the subscriber at station AI causes the release of relays H2, I55, I24, I25 and I22; and the release of the connection by the subscriber at station A2 causes the release of relays 2I2, 2Ifi, 224, 226 and 223. With the connection released at both ends the trunk may be seized at either end. If the trunk is released at one end and again seized at that end before being released at the other end, the battery feed relay (I25 or 225) at the seized end of the trunk operates in series with the bridge relay (221 or I21) at the opposite end of the trunk; and the connection thus originated at the released end of the trunk may becompleted without awaiting the release of the connection at the other end.

If, in a case when the trunk T is seized at both ends, the bridge relay I 21 operates and locks before seizure of the trunk in office O-I effects the operation of relay I29, then the trunk T will be extended by the operation of relays H8 and I 30 to the incoming selector ISI for use in completing the call incoming from office 0-2; in this case the failure of relay I25 to operate causes "the operation oftrelay' I 23 to connect abusy tone :over conductor ID! to the line of the calling subscribenin ofliceO-Jas above described. But

if the bridge relay-22'! operates and locks before *seizure' of the trunkin ofiice 0-2 effects the .operation of relay 229, then the trunk T will be extended by the operation of relays 2l8 and 235! .to the incoming selector 182 for use in completing thecall incoming from office OI; and

in this case the failure of relay 225 to operate results in the operation of relay 223 to connect a busy tone over conductor 201 to the line of the calling subscriber in ofiice O2.

What is claimed is: .1. In atelephone system, two offices, a trunk between said ofiices, automatic switches in each end, means at one end of the trunk for opening .the bridge across that end of the trunk in response to seizure of the trunk at the other end, and means at said other end of the trunk for opening the bridge across said other end of the trunk in response to seizure of the trunk at said one end.

2. In a multi-ofiice telephone system, a trunk for completing calls between two of the ofiices,

automatic means in each of said offices for seizingsaid trunk, and means in each of said oliices responsive to the simultaneous seizure of said trunk in both of said oihces for connecting a busy tone to one of the talking conductors of each of the switches by which said trunk is seized.

3. In a telephone system, subscribers lines, a two-way trunk'for use in completing telephone calls, an automatic switch at each end of said trunk for use in extending connections from calling lines to said trunk, and means at one end of said trunk effective upon seizure of the trunk at said one end after seizure of the trunk at the other end for connecting a busy tone to one of the talking conductors of the switch by which the trunk is last seized.

4. In a telephone system, subscribers lines, a two-way trunk for use in completing telephone calls, an automatic switch at each end of said trunk for use in extending connections from calling lines to said trunk, and means at each end of said trunk for connecting busy tone to one of the talking conductors of the switch by which 'said trunk is seized, each of said means being operatively effective in response to the seizure of said trunk at the end with which the means is associated after seizure of the trunk at the other 5. In a telephone system, a trunk, an automatic switch at each end of said trunk for seizing said trunk, a bridge normally connected across each end of said trunk for responding to seizure of the trunk at the opposite end, each of .said bridges being free from battery and ground and each comprising a relay for responding to seizure of the trunk at the opposite end, a switch of said switches which is individual to that end of-the trunk.

6. In a telephone system, a trunk, an automatic switch at each end of said trunk for seizing .-said.trunk, a bridge normally connected across ram 1,221

each end of said trunk forresponding toseizure of the trunk at the opposite end, each of said bridges being free from battery and ground and each comprising a relay for responding to seizure of the trunk at the opposite end, means responsive to seizure of said trunk at one end for disconnecting the bridge normally connected across that end, means responsive to seizure of the trunk at the other end for disconnecting the bridge normally connected across said other end, a switch individual to each end of said trunk for use in completing calls incoming over said trunk, and means at each end of said trunk rendered effective by the operation of the relay in the bridge at that end for extending said trunk to the one of said switches which is individual to that end of the trunk. Y

7. In a telephone system, subscribers lines in each of two ofiices, a trunk for completing calls between said oiiices, an automatic switch in each of said oiiices for seizing said trunk, a relay winding bridged across one end of said trunk for operation in response to seizure of said trunk at the distant end, a switch at said one end of the trunk and individual thereto, means responsive to the operation of said relay for extending the conductors of said trunk to said individual switch, and means responsive to the seizure of said trunk at said one end after seizure of the trunk at said distant end for connecting a busy tone to one of the talking conductors of the switch by which said trunk is last seized without interfering with the incoming call extended to said individual switch.

In a multi oifice telephone system, a trunk between two oflices, subscribers lines in each ofiice, automatic switches in each of said oiliccs for seizing said trunk and for extending calling lines to said trunk, a switch in one of the oifices individual to said-trunk for use in completing calls incoming over said trunk, a relay winding bridged across said trunk in said one of the ofiices for responding to seizure of said trunk in the other of said oifices, means responsive to the operation of said relay for extending the conductors of said trunk to said individual switch. means responsive to the seizure of said trunk and the extension of a calling line to said trunk by one of the automatic switches in said one of the oifices subsequent to the operation of said relay for transmitting a busy tone to the calling subscriber, and means responsive to the opening of the line of said calling subscriber for disconnecting the switch by which said trunk was last seized without interfering with the completion of the incoming connection through said individual switch.

9. In a telephone system, subscribers lines, a two-way trunk, an automatic switch at each end of said trunk for seizing and appropriating said trunk for use in completing calls over said trunk, means at each end of said trunk responsive to seizure for closing a talking connection between the talking conductors of the seizing switch and the conductors of said trunk, and means responsive to the simultaneous seizure of said trunk by both of said switches for preventing the closure of said talking connections.

10. In a telephone system, a first office, a second office, a two-way trunk between said offices, automatic switches in each of said ofiices for seizing said trunk, a relay normally bridged across the trunk in the first office, a relay normally bridged across the trunk in the second ofiice, means responsive to seizure of the trunk in the second office for disconnecting the bridging relay in that oince and for causing the operation of the bridging relay in the first ofiice, and means responsive to said operation of the bridging relay in said first oilice for disconnecting said bridging relay from said trunk.

11. In a telephone system, a first ofiice, a second oifice, a two-way trunk between said ofiices, automatic switches in each of said ofiices for seizing said trunk, a relay normally bridged across the trunk in the first office, a relay normally bridged across the trunk in the second oiiice, means responsive to seizure of the trunk in either oil-ice for disconnecting the bridging relay in that ofiice and for causing the operation of the bridging relay in the other oifice, and means in each office responsive to the operation of the bridging relay in the same oflice due to seizure of the trunk in the other oifice for disconnecting the operated bridging relay from the trunk.

12. In a telephone system, a trunk, means comprising an automatic switch at each end of said trunk for seizing said trunk, a relay at each end of said trunk, the winding of each of the relays being normally bridged across the conductors of the trunk, a selector individual to each end of the trunk for use in completing calls incoming over the trunk, and means at each end or" said trunk rendered effective by the operation of the relay at the same end for extending the trunk to the associated individual selector.

13. In a telephone system, a two-Way trunk, switching means at one end of the trunk for seizing said trunk for use in completing an outgoing call, a selector at the other end of the trunk for use in completing incoming calls, a relay at said other end of the trunk, the winding of said relay being bridged across the trunk for energization in response to the seizure of said trunk at said one end, and means rendered effective by the operation of said relay for extending the conductors of said trunk to said selector.

14. In a telephone system, a two-way trunk, switching means at one end of the trunk for seizing said trunk for use in completing an outgoing call, a selector at the other end of the trunk for use in completing incoming calls, a relay at said other end of the trunk, the winding of said relay being bridged across the trunk for energization in response to the seizure of said trunk at said one end, and means rendered effective by the operation of said relay for disconnecting the conductors of the trunk from the winding of said relay and for connecting the conductors of the trunk to said selector.

15. In a telephone system, a two-way trunk, means at each end of the trunk for seizing said trunk for use on an outgoing call, a selector individual to each end of the trunk for use in completing incoming calls, a relay at each end of the trunk, the winding of each of the relays being normally bridged across said trunk, means responsive to seizure of the trunk at one end for disconnecting the bridging relay at that end and for causing the operation of the bridging relay at the other end, and means responsive to the operation of the bridging relay at one end of the trunk for operatively connecting the trunk to the selector individual to that end of the trunk.

167 In a telephone system, a two-way trunk, means at one end of the trunk for seizing said trunk for use in completing an outgoing call, a switch at the other end of the trunk for use in completing incoming calls, a relay at said other end of the trunk, the winding of said relay being bridged across the trunk for energization in response to the seizure of the trunk at said one end, an impulse receiving relay for said switch, and means operatively responsive to the energization of said bridging relay for connecting the conductors'of said trunk to the windings of said impulse receiving relay.

1?. In a multi-oflice telephone system, subscribers lines in each of two offices, an impulse sending device for each line, a trunk between said offices, a bridge comprising the winding of a relay normally connected across each end of the trunk, means in each ofiice for connecting a calling line to the trunk, a selector individual tothe trunk in each of the offices for use in completing incoming calls, means efiective upon seizure of the trunk in one ofiice for causing the operation of the bridging relay in the other ofiice, means operatively responsive to the operation of the bridging relay in one office for extending the conductors of the trunk to the selector which is individual to the trunk in that ofiice, and means associated with the trunk in each ofiice for repeating impulses received from a calling line over the trunk to selectively operate the individual selector in the other oflice.

18. In a telephone system, subscribers lines, an impulse sending device for each line, a twoway trunk, switching means at one end of the trunk for connecting a calling line to said trunk, a selector at the other end of said trunk for use in completing incoming calls, a relay at said other end of the trunk, the winding of said relay being bridged across the trunk for energization in response to the connection of a calling line to said one end of the trunk, means rendered efiective by the operation of said relay for connecting the conductors of said trunk to said selector, and means at said one end of the trunk for receiving impulses from a calling line and for repeating the impulses over the trunk to selectively operate said selector.

JOHN BAUMFALK. 

